eigrpd: fix byte order in Hello authentication decode

[TristanInSec]

eigrp_hello_authentication_decode() compares md5->auth_type and md5->length directly against host-order constants, but these struct fields are stored in network byte order (the send path uses htons() at eigrp_packet.c:1248). On little-endian hosts the comparisons never match, so authentication is silently skipped.

Wrap both fields with ntohs() to match the encoding.

Signed-off-by: Tristan Madani tristan@live.fr

[greptile-apps]

Greptile Summary

This PR fixes a network byte order bug in eigrp_hello_authentication_decode(): the auth_type and length fields of received TLV structs are in network byte order (the send path uses htons() at eigrp_packet.c:1248), but were compared directly against host-order constants. On little-endian hosts this caused all authenticated EIGRP hello packets to be silently dropped.

The fix — wrapping both fields with ntohs() — is minimal and correct. Note that eigrp_hello_parameter_decode() (line 110) and eigrp_sw_version_decode() (line 221) have the same pattern (tlv->length < sizeof(...) without ntohs()), which are pre-existing bugs not addressed here.

Confidence Score: 5/5

Safe to merge — the fix is minimal, correct, and directly mirrors the encoding used in the send path.

The change is a four-line, mechanical byte-order correction with no logic complexity. The ntohs() calls match the htons() in eigrp_add_authTLV_MD5_to_stream / eigrp_add_authTLV_SHA256_to_stream. No new code paths or data structures are introduced. All remaining notes are about pre-existing issues outside the diff.

No files require special attention; the only changed file applies the fix correctly.

Important Files Changed

Filename	Overview
eigrpd/eigrp_hello.c	Adds ntohs() around md5->auth_type and md5->length in eigrp_hello_authentication_decode() — correct fix matching the htons() encoding in the send path.

Sequence Diagram

sequenceDiagram
    participant Wire as Network Wire
    participant Recv as eigrp_hello_receive
    participant Decode as eigrp_hello_authentication_decode
    participant Check as eigrp_check_md5_digest

    Wire->>Recv: Hello packet (fields in network byte order)
    Recv->>Recv: ntohs(tlv_header->type) → EIGRP_TLV_AUTH
    Recv->>Decode: pass raw tlv_header pointer
    Note over Decode: BEFORE fix: auth_type compared without ntohs,<br/>always fails on little-endian hosts
    Note over Decode: AFTER fix: ntohs(md5->auth_type) == EIGRP_AUTH_TYPE_MD5
    Note over Decode: AFTER fix: ntohs(md5->length) validates TLV size
    alt auth type matches and length valid
        Decode->>Check: eigrp_check_md5_digest or eigrp_check_sha256_digest
        Check-->>Decode: pass or fail result
        Decode-->>Recv: result (1 = ok, 0 = drop)
    else no match
        Decode-->>Recv: 0 drop packet
    end
    Recv->>Recv: if 0 return early, else continue

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_{Reviews (1): Last reviewed commit: "eigrpd: fix byte order in Hello authenti..." | Re-trigger Greptile}

[mjstapp]

the greptile review notes another couple of places that may have the same issue: can you add those to the PR?

[TristanInSec]

Good catch. Added ntohs() to the parameter, software version, and peer termination decode functions in the latest push.

[mjstapp]

ah: please rebase and squash/fixup the second commit into the first, no need to have two commits there (and the commit-format checker is flagging it anyway)

[mjstapp]

code looks good, thanks (still need the commit fixes before merging)

[donaldsharp]

@Mergifyio backport stable/10.6 stable/10.5 stable/10.4 stable/10.3 stable/10.2 stable/10.1 stable/10.0

[mergify]

backport stable/10.6 stable/10.5 stable/10.4 stable/10.3 stable/10.2 stable/10.1 stable/10.0

✅ Backports have been created

Details

• #21590 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.6
• #21591 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.5
• #21592 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.4
• #21593 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.3
• #21594 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.2
• #21595 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.1
• #21596 eigrpd: fix byte order in Hello authentication decode (backport #21545) has been created for branch stable/10.0